Reading Waveform on Set

How do you locate an object in the waveform monitor? I know that the you need to look on the x-axis, but how about its position on the y-axis? For instance, if I need to know the IRE of my subject’s shirt pocket, do I just eyeball the object’s brightness on my field monitor and then based on how the brightness looks like, do a good guess on where it is on the waveform? Thanks! God bless!